The present invention relates to a circuit for scan conversion which converts interlaced scanned picture signals into progressive scanned picture signals by deinterlacing, and more particularly to a circuit for scan conversion of picture signals using motion compensation, which is hardly susceptible to flicker artifact or deterioration in resolution and suitable for realization of low-cost scan conversion ensuring high picture quality.
Recently, increasing numbers of personal computers and television sets display their pictures by progressive scanning with a view to ensuring high picture quality.
However, most of the available video software products consist of signals deriving from interlaced scanning of objects. Accordingly, they require processing to convert interlaced scanned signals into progressive scanned signals.
Many ideas using motion-adaptive interpolation have been proposed for this signal processing. Motion-adaptive interpolation means generation of signals on scanning lines skipped by interlaced scanning to convert them into progressive scanned signals by adaptively varying, according to the motions of pictures, the mixing ratio of signals suitable for still pictures generated by field insertion and signals suitable for moving pictures generated by intra-field interpolation.
However, the poor vertical resolution of interpolation signals for moving pictures invites a problem that certain motions are inevitably subject to blurring or flicker artifact.
To solve this problem, research and development attempts are now under way to achieve conversion into progressive scanning by motion compensation. Such attempts are intended to generate interpolation signals suitable for moving pictures from signals resulting from the shifting of signals in the previous field to the position of the current field according to motion vectors and from signals of the current field, and thereby to realize conversion into high quality progressive scanning which can hardly be attained by the conventional motion-adaptive interpolation.
This motion-compensated scan conversion, however, requires detection of the motion vectors of pictures for the motion compensation process. Also, if motion vectors are erroneously detected, very obstructive interference will arise, inviting extreme deterioration of picture quality.
Therefore, application of a motion-compensated scan conversion circuit to a television receiver or a data processing terminal for home use would involve many unsolved problems including efficient detection of motion vectors and control of picture quality deterioration ensuing from erroneous detection.
The present invention, attempted in view of the above-noted problems, is intended to provide a motion-compensated scan conversion circuit ensuring high picture quality at low cost.
To achieve this object, the invention realizes a low-cost scan conversion circuit for high quality picture signals by the technical means described below.
Regarding the detection of motion vectors, first the already detected vectors around the current block are used as reference vectors, and that with the least estimated motion error is set to be the representative vector of the current block. Next, blocks whose estimated motion errors exceed the threshold are subjected to re-search by tree search or block matching to detect motion vectors. Then, miniblocks resulting from the horizontal and vertical division of blocks are subjected to miniblock division search using the motion vectors of surrounding blocks. Further, the motion vector of each pixel is detected by smoothing the motion vectors of these miniblocks. This means for signal processing by motion vector detection can dramatically reduce the quantity of computation needed for motion vector detection to a very small fraction of the current volume, a quotient of division by tens or even hundreds.
Regarding the control of picture quality deterioration ensuing from erroneous detection of motion vectors, the reliability of motion compensation is checked by taking note of the signal continuity between the current pixels on interpolated scanning lines generated by motion compensation and pixels on interlaced scanning lines above and underneath them, and comparing the former pixels with the latter. In a region where reliability is determined to be insufficient, the generation of interpolation signals by motion compensation is stopped. It has been found that where re-searching is frequently done in the aforementioned procedure of motion vector detection, the accuracy of motion vectors is also poor. In view of this finding, signal processing is carried out to vary the threshold of pixel comparison according to the frequency of this re-searching. Further signal processing is performed to restrict the generation of interpolation signals by motion compensation to the region of high vertical frequency components of pictures or to a prescribed speed range of motion vectors. These means for signal processing can very efficiently eliminate picture quality deterioration intrinsic to motion compensation attributable to erroneous detection of motion vectors.